Hello
I am using SharePoint Foundation 2010 and I am trying to validate a column in a list that contains a telephone number. I don't want to restrict the users to a length as the number they enter could be for the UK or possibly overseas. What I would like to do is make the validation formula only allow numbers and hyphens in the field. For instance a user could enter 0121-000-0000 or 01-21-000-0000-00 but they cannot enter text and they cannot enter numbers without the hyphens.
Also I would like the validation to allow the field to remain empty, since it is not a required field. I've played around and tweaked a few validation formulas I have found but I don't really understand what I am doing so they don't work.
Thanks
